1. Field of the Invention
This invention relates to integrated circuits and, more particularly, to methods and apparatus for providing a multiple layer programmable layout which may be used for the identification of a version of an integrated circuit product.
2. History of the Prior Art
Historically, manufacturers have identified integrated circuits used in personal computers and other products by a code stored in read only memory within the integrated circuits. Typically, this code includes two bytes of information. The first byte is used to identify the manufacturer of the component, and the second byte is used to identify the component. Many computer application programs use this information to determine various details of program operation utilizing that component. Recently, at least one manufacturer has enlarged the available space in read only memory within the integrated circuit and added additional information to the identification information. One piece of information which has been added is a version number. The information is used by the manufacturer for various testing operations. It is very useful to be able to determine the particular version of a product so that tests especially designed for that version of the product may be run. For example, changes in individual masking layers often cause the different versions of a product to consume different amounts of power during test procedures. Unless the particular version of an integrated circuit is known, appropriate full power tests cannot be conducted on the part. Since the version number is presently used only for testing, reading this version number has required a high accessing voltage which is not available in the typical use of the product. Thus the version information is not available during the normal user operations.
To date each entirely new version of a product is given a new version number. The number is typically stored by providing one of two voltage levels at each of a number of external terminals. The particular voltage level represents one of the bits of the version number. The voltage levels are typically provided through hard wired connections to the voltage sources. These connections are usually formed in one of the metal conducting layers.
Changes are made to integrated circuits by changing the mask by which a particular layer is formed. Changes are often made in only a single masking layer because these changes are easy and inexpensive to make. However, prior art circuits typically required that changes be made in a plurality of masking layers in order to change a version number. Because changing a number of masking layers is quite expensive, a version number has not usually been changed in prior art products where only a single layer of the masking for forming the integrated circuit has changed. Consequently, the version number has remained the same until a major change to the integrated circuit is necessary. As is obvious, this is not desirable since appropriate testing cannot be conducted without knowing the layout of each of the layers. A version number is the easiest way to know the makeup of the layers.